An important area of application for methods and mass airflow sensors is the measurement of mass airflows in the intake air ducts of modern internal combustion engines. Namely, precise control of the combustion in such internal combustion engines requires the amount of air drawn in through the intake air duct to be precisely measured, in order to maintain an optimal fuel-to-air ratio during combustion.
For the measurement of such a mass airflow, multiple heated wire or hot film mass airflow meters are used. The basis of the way in which these sensors work is that a mass airflow cools a heated body down to an extent which corresponds to the magnitude of the mass airflow around the body. Accordingly, a current flowing through a heating resistor is controlled to maintain the heating resistor at a constant temperature above the temperature of the mass airflow. The heating current required to achieve this represents a very exact, albeit non-linear, measure of the mass airflow.
Provided that the air in an intake air duct always flows in one direction only, these sensors work with adequate precision. However, with internal combustion engines, operating conditions can arise in which air in the intake air duct of the internal combustion engine is subject to pulsations. These pulsations can become so strong that a backflow of the air occurs, in the reverse of the normal intake direction. However, the measurement principles described above, using heated wire or hot film mass airflow meters, only permit the magnitude of a mass airflow to be determined, but not its direction. In the case of pulsations, this can lead to a backflow being measured as an inflow of intake air, which makes control of the internal combustion engine significantly more difficult.
One possible way of recognizing such backflows consists in the use of two sensors spaced apart in the direction of the flow, or one sensor with two sensory elements spaced apart in the direction of the flow, so that by comparing their values the presence of a backflow can be inferred. However, such arrangements have a comparatively complicated construction, and demand costly assembly in an intake air duct.
DE 43 42 481 C2 describes a method for the measurement of the air mass drawn into an internal combustion engine, using a temperature-sensitive measurement sensor in its intake air duct, whereby, from average load states of the internal combustion engine onwards, a supplementary heating element, located downstream from the measurement sensor in the induction direction, is heated up to produce an error-compensating effect on the measurement sensor. This method requires the additional installation of the supplementary heating element into the intake air duct, which increases the manufacturing costs.